Single-mode polymer waveguide connector

ABSTRACT

Waveguide connectors include a ferrule having first alignment features. A polymer waveguide has one or more a topclad portions, each with a waveguide core, second alignment features fastened to the first alignment features, and underclad portion that is thicker than the one or more topclad portions. The polymer waveguide has a higher coefficient of thermal expansion than the ferrule and is fastened to the ferrule under tension.

BACKGROUND

Technical Field

The present invention relates to optical waveguides and, moreparticularly, to a single-mode polymer waveguide connector.

Description of the Related Art

Waveguides are used to transport, e.g., optical signals over largedistances with very low losses. Waveguides employ, e.g., a differencebetween a refractive index for an internal medium called “core” and anexternal medium called “clad”. In the example of fiber optics, thetransport medium “core” is made of a higher refractive index glass andthe external medium “clad” is made of a lower refractive index glass.These two glass layers are surrounded by sheath, shielding, or air. Whenan optical signal in the inner core layer hits the boundary between coreand clad, it is internally reflected instead of escaping from corelayer. As a result, optical fibers can be used to transport verylow-loss signals across long distances.

Optical fibers used for waveguides are thin, flexible, and frequentlymade of silica glass, but may also be made from, e.g., fluoride glass,phosphate glass, chalcogenide glass, or crystal materials such assapphire. Appropriate materials are selected in accordance with desiredrefractive properties. Transmissions over optical fiber suffer from lessloss and electromagnetic interference relative to metal wires. Inaddition, since information propagates through optical fibers at thespeed of light, latency is decreased over large distances using opticalcommunications. Some fibers support many transverse transmission modesand are called multi-mode fibers, whereas others support a single modeand are called single-mode fibers. Single-mode fibers are frequentlyused for long-distance links, as multi-mode fibers are susceptible tomodal dispersion over long distances due to slightly differenttransmission speeds between the different modes.

Polymer materials exhibit favorable properties for use in opticalwaveguides. Polymers provide good optical properties and are costeffective and easy to fabricate. Polymers are furthermore compatiblewith printed circuit board manufacturing processes due to a resilienceagainst solder reflow and lamination processes, such that polymerstructures can be formed directly on printed circuit boards alongsidesemiconductor-based components. Polymer waveguides are used forhigh-density optical interconnects in fiber-optic communications,optronics, and other light-based technologies. Waveguide connectors areused to connect between separate polymer waveguides and between polymerwaveguides and glass fibers.

Existing polymer waveguide connectors are difficult to install underprecise positioning requirements. One such connector is the polymermechanical transfer (PMT) connector which is used as a multimode polymerwaveguide connector. These connectors are difficult to assemble withpositioning errors of under a few micrometers and are simply notfeasible for positioning errors of less than a micrometer. Positioningerrors can lead to loss of signal from, e.g., reflections that occur atimperfect junctions.

Although efforts have been made to improve positioning for polymerwaveguide connectors, difficulties arise in fabrication. The initialpositioning error of, e.g., a positioning groove on a waveguide may beunder a micrometer due to precise lithography fabrication, but thepolymers use generally have high coefficients of thermal expansion(CTEs), which causes the polymer to shrink or expand as the temperaturechanges.

SUMMARY

A waveguide connector includes a ferrule comprising first alignmentfeatures and a polymer waveguide. The polymer waveguide includes one ormore a topclad portions, each comprising a waveguide core, secondalignment features fastened to the first alignment features, and anunderclad portion that is thicker than the one or more topclad portions.The polymer waveguide has a higher coefficient of thermal expansion thanthe ferrule and is fastened to the ferrule under tension.

A waveguide connector includes a ferrule having ferrule studs. Awaveguide includes one or more a topclad portions, each having awaveguide core, grooves that are fastened to the first alignmentfeatures with an adhesive, and an underclad portion is thicker than theone or more topclad portions. The waveguide cores are vertically alignedto within about 1 μm of one another. The waveguide cores have lateralmispositioning deviations from a design spacing of less than about 1 μm.The polymer waveguide has a higher coefficient of thermal expansion thanthe ferrule and is fastened to the ferrule under tension.

A waveguide connector includes a mechanical transfer connector ferrulecomprising ferrule studs. The ferrule has a coefficient of thermalexpansion of about 1 ppm/° C. A polymer waveguide has a coefficient ofthermal expansion of about 50 ppm/° C. and includes one or more atopclad portions, each comprising a waveguide core and each having athickness between about 23 μm and about 24 μm, grooves fastened to thefirst alignment features with an adhesive, and an underclad portionhaving a thickness of about 50 μm. The waveguide cores have a spacing ofabout 250 μm, are vertically aligned to within about 1 μm of oneanother, and have lateral mispositioning deviations from a designspacing of less than about 1 μm. The polymer waveguide has a highercoefficient of thermal expansion than the ferrule and is fastened to theferrule under tension.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of a waveguide connector inaccordance with the present principles; and

FIG. 2 is a block/flow diagram of a method of forming a waveguideconnector in accordance with the present principles; and

FIG. 3 is a cross-sectional diagram of a conventional waveguideconnector.

DETAILED DESCRIPTION

Embodiments of the present invention provide polymer waveguides withprecise positioning, having positioning errors below one micrometer. Toaccomplish this, the differing coefficients of thermal expansion (CTEs)of a waveguide material and a ferrule material are exploited by heatingthe two structures until they align, fastening the structures together,and letting them cool, thereby creating a tension in the polymerwaveguide that precisely aligns the individual waveguides in theferrule. By increasing the thickness of the bottom cladding of thepolymer waveguide and by removing the backfilm that is normally used forsupport and positioning, the waveguide positioning may be made moreconsistent.

The functioning of a waveguide relies on total internal reflection oflight—or other electromagnetic radiation—at a boundary. Waveguides atoptical frequencies often take advantage of a difference in refractiveindex between two materials at the boundary. Fiber can be multi-mode orsingle-mode, referring to the propagation modes of the light as itpasses through the waveguide. In the case of multi-mode fibers, multipledifferent transverse modes (i.e., multiple different light paths throughthe waveguide) can exist simultaneously in the relatively largewaveguide core, where the core has a diameter that is much larger thanthe wavelength of the light carried. By contrast, in a single-modefiber, only one transverse mode, called a zeroth mode or a fundamentalmode, exists. This advantageously inhibits modal dispersion and providessuperior fidelity of signals over long distances. In a single-modefiber, a wavelength division multiplex (WDM) technology is often usedfor a broadband communication. When WDM is applied, multiple differentfrequencies of light are transmitted along one single-mode fiber, eachpropagating along the fiber in the fundamental mode. When dealing withsingle-mode optical fiber, an intuitive geometric interpretation for thepropagation of light within the fiber is unavailable, with the behaviorof the propagation being modeled instead using the Helmholtz equation.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, the placement of a polymerwaveguide 104 on a ferrule 102 is shown. A lid 106 is used to place andhold the waveguide 104. It is contemplated that the waveguide 104 mayhave an exemplary CTE of about 50 ppm/° C., while the ferrule 102 mayhave an exemplary CTE of about 1 ppm/° C. The waveguide 104 includes aset of waveguide cores 114, a set of narrow grooves 111 between cores,and two wide grooves 110 on both sides of the core array. The ferrule102 includes two wide studs 112 and a set of narrow studs 116. At roomtemperature (e.g., 25° C.), if the distance between the two grooves 110is about 3 mm, this distance would be an exemplary 3 μm smaller than thedistance between the two studs 112 at room temperature. At a temperatureof 45° C., however, the differeing CTEs of the two structures results inthe grooves 110 aligning with the studs 112, with the waveguide cores114 also aligning with the narrow studs 116. It is specificallycontemplated that the ferrule 102 may be part of, e.g., a mechanicaltransfer (MT) connector, but any appropriate connector may be usedinstead.

Optical connectors terminate an end of the optical fiber and provide forrapid connection and disconnection. By aligning the fibers of twosections of optical cable, the connectors ensure an easy connection andallow light to pass with little loss. Ideally the connectors have someform of locking ability that maintains a strong connection and preventsfibers in respective connectors from moving relative to one another.Maintaining good alignment is important for minimizing return loss,which occurs at discontinuities in the connection. Even small deviationsin positioning and alignment can create significant return losses. Thepresent embodiments bring cores 114 of respective connectors intoalignment with very low deviation from the expected positions.

The MT connector is a multi-fiber connector that is often used forribbon cables. It is used in, for example, preterminated cableassemblies and cabling systems. In particular, the MT connector allowsmultiple single-mode fibers to connected in parallel, such that onefiber ribbon cable will include multiple glass fibers and therebyprovide increased transmission bandwidth. Connection strength isprovided by latches on the connector that lock into place on a matedplug using a spring mechanism. Guide pins are used to aid in alignmentof the ferrules 102 and removable housings may be employed formodularity. While this provides good mechanical alignment between tworespective connectors, manufacturing imperfections can still result inmisalignment between the small waveguide cores.

The polymer waveguide 104 is formed by forming waveguide cores on alower refractive index under cladding polymer layer. The cores areformed by depositing, e.g., a higher refractive index polymer materialusing a photo lithography method or any other appropriate depositionmethod and patterning the core material to form waveguides of thedesired shape. A lower index upper cladding polymer material is thendeposited over the cores. The optical signal is confined by internalreflection to the waveguide core material at the interface between thewaveguide core and the upper and lower cladding material. Single-modeglass fibers often have core diameters from about 5 to about 11 μm. Thecorresponding single-mode polymer waveguides also have a few crosssectional area of a few micrometers.

During placement, the waveguide 104 is heated to a temperature thatcauses an expansion of the waveguide 104, allowing it to align with thestuds 116 and 112 of the ferrule 102 as described above. A glue 108 isapplied to respective grooves 110 of the waveguide 104 and studs 112 ofthe ferrule 102. The glue 108 may be, e.g., an ultraviolet-cured gluethat is then exposed to ultraviolet light, locking the sides of thewaveguide 104 in place. Although it is specifically contemplated that aglue may be used, any other appropriate form of bonding may be employedinstead. As the waveguide 104 cools, the waveguide 104 is prevented fromshrinking accordingly and a tension is created within the waveguide 104that pulls the polymer waveguide 104 flat and brings each waveguide core114 into a precise position within the narrow studs 116.

Referring now to FIG. 3, a cross-sectional diagram of a conventionalconnector with a backfilm and a thin underclad layer is shown. Inconventional connector construction, a hard back film is used on arelatively thin polymer waveguide. The hard back film 304 is glued to athinner waveguide 302. The soft waveguide layer 302 and glue layer arethen sandwiched between the hard back film 304 and studs of the ferrule112/116, causing these layers to buckle and become distorted uponcooling. Binding the waveguide material to the backfilm causes the CTEdifference between those structures to create distortions andmisalignment of waveguide cores in the material.

To address this, the present embodiments omit the back film entirely andinstead increase a thickness of the waveguide 104. The thicker waveguide104 provides structure and consistency in CTE. A glass lid 106 is thenapplied directly to the waveguide 104 to apply pressure while theultraviolet glue 108 sets. The thickness of the underclad portion of thewaveguide 104 may be, for example, about 50 μm—increased relative toconventional waveguides which have the underclad portion with athickness of about 20 μm. This thickness provides stability of a precisecore postion during assembly, easy manipulation of the waveguide 104,and increased physical strength of the waveguide 104 after assembly. Inone exemplary embodiment, the underclad portion of the waveguide 104 hasa thickness of about 50 μm while the topclad portion that includeswaveguide cores 114 has a thickness of about 23 to 24 μm. In thisexemplary embodiment, a spacing between cores 114 is about 250 μm.

Omitting the back film and using a waveguide 104 provides superioralignment of the waveguide cores 114, with experimentally demonstratedpositioning errors of less than 1 μm. Improvements are shown in height,lateral, and absolute misalignment values. The thickness of thewaveguide 104 does not decrease the CTE of the material, which is usedto provide precise alignment of the waveguide cores 114.

Referring now to FIG. 2, a method for constructing a connector is shown.Block 202 heats the polymer waveguide 104 and the ferrule 102 until thegrooves 110 align with studs 112. As noted above, the polymer waveguide104 starts with, e.g., a smaller width between grooves 110 than thewidth between the studs 112 of the ferrule 102. Due to CTE mismatchbetween the materials of the polymer waveguide 104 and the ferrule 102,the polymer waveguide 104 will expand at a different rate per degree oftemperature change, such that at some temperature the widths will beequal. Block 204 applies, e.g., an ultraviolet curing glue 108 to thegrooves 110 and/or the studs 112 and block 206 positions the polymerwaveguide 104 on the ferrule 102, aligning the grooves 110 and the studs112.

Block 208 applies pressure to the polymer waveguide 104 using, e.g., theglass lid 106. Block 210 cures the glue by applying, e.g., ultravioletlight. This locks the grooves 110 and the studs 112 together. Block 212then removes the pressure from the polymer waveguide 104 and allows thepolymer waveguide 104 and the ferrule 102 to cool. As they cool, thepolymer waveguide 104 attempts to contract more quickly than the ferrule102, creating a tension within the material of the polymer waveguide104. This tension pulls the cores 114 precisely into position.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of” for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Having described preferred embodiments of a single-mode polymerwaveguide connector (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

1. A waveguide connector comprising: a ferrule comprising firstalignment features; a polymer waveguide, comprising: one or more atopclad portions, each comprising a waveguide core; second alignmentfeatures fastened to the first alignment features; and an undercladportion that is thicker than the one or more topclad portions, whereinthe polymer waveguide has a higher coefficient of thermal expansion thanthe ferrule and is fastened to the ferrule under tension.
 2. Thewaveguide connector of claim 1, wherein the first alignment featurescomprise ferrule studs and the second alignment features comprisewaveguide grooves.
 3. The waveguide connector of claim 1, wherein thepolymer waveguide has a coefficient of thermal expansion of about 50ppm/° C. and the ferrule has a coefficient of thermal expansion of about1 ppm/° C.
 4. The waveguide connector of claim 1, wherein the undercladportion has a thickness greater than about 20 μm.
 5. The waveguideconnector of claim 4, wherein the underclad portion has a thickness ofabout 50 μm.
 6. The waveguide connector of claim 4, wherein the one ormore top clad portions have a thickness between about 23 μm and about 24μm.
 7. The waveguide connector of claim 4, wherein a spacing betweenwaveguide cores is about 25 μm.
 8. The waveguide connector of claim 1,wherein the waveguide cores are vertically aligned to within about 1 μmof one another.
 9. The waveguide connector of claim 1, wherein thewaveguide cores have lateral mispositioning deviations from a designspacing of less than about 1 μm.
 10. The waveguide connector of claim 1,wherein the second alignment features are fastened to the firstalignment features with an adhesive.
 11. The waveguide connector ofclaim 1, wherein the ferrule is configured as part of a mechanicaltransfer connector.
 12. A waveguide connector comprising: a ferrulecomprising ferrule studs; a waveguide, comprising: one or more a topcladportions, each comprising a waveguide core, wherein the waveguide coresare vertically aligned to within about 1 μm of one another and whereinthe waveguide cores have lateral mispositioning deviations from a designspacing of less than about 1 μm; grooves fastened to the first alignmentfeatures with an adhesive; and an underclad portion that is thicker thanthe one or more topclad portions, wherein the polymer waveguide has ahigher coefficient of thermal expansion than the ferrule and is fastenedto the ferrule under tension.
 13. The waveguide connector of claim 12,wherein the polymer waveguide has a coefficient of thermal expansion ofabout 50 ppm/° C. and the ferrule has a coefficient of thermal expansionof about 1 ppm/° C.
 14. The waveguide connector of claim 12, wherein theunderclad portion has a thickness greater than about 20 μm.
 15. Thewaveguide connector of claim 14, wherein the underclad portion has athickness of about 50 μm.
 16. The waveguide connector of claim 14,wherein the one or more top clad portions have a thickness between about23 μm and about 24 μm.
 17. The waveguide connector of claim 14, whereina spacing between waveguide cores is about 250 μm.
 18. The waveguideconnector of claim 12, wherein the ferrule is configured as part of amechanical transfer connector.
 19. A waveguide connector comprising: amechanical transfer connector ferrule comprising ferrule studs, theferrule having a coefficient of thermal expansion of about 1 ppm/° C.; apolymer waveguide having a coefficient of thermal expansion of about 50ppm/° C., comprising: one or more a topclad portions, each comprising awaveguide core and each having a thickness between about 23 μm and about24 μm, wherein the waveguide cores have a spacing of about 250 μm, arevertically aligned to within about 1 μm of one another, and have lateralmispositioning deviations from a design spacing of less than about 1 μm;grooves fastened to the first alignment features with an adhesive; andan underclad portion having a thickness of about 50 μm, wherein thepolymer waveguide has a higher coefficient of thermal expansion than theferrule and is fastened to the ferrule under tension.